Mother joins aristocrat in complaint (6)
I believe the answer is:
malady
'complaint' is the definition.
(malady is a kind of health problem)
'mother joins aristocrat' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'joins' says to put letters next to each other.
'aristocrat' becomes 'lady' (lady is a kind of aristocrat).
'ma'+'lady'='MALADY'
'in' acts as a link.
(Other definitions for malady that I've seen before include "Sickness or disease" , "affection" , "Illness or disease" , "hay fever, perhaps" , "Ailment; illness" .)